Two women, including a serving police officer, have been arrested as part of Scotland Yard's investigation into the "plebgate" affair involving the former Tory chief whip Andrew Mitchell.

A 37-year-old police constable from the diplomatic protection group, which is responsible for protecting diplomatic and government officials, was arrested at her workplace on suspicion of misconduct in a public office, the Metropolitan police said.

A 46-year-old woman was also arrested at her workplace on suspicion of assisting the commission of an offence of misconduct in a public office.

The women have been taken to separate police stations – one in London, one outside London – where they remain in custody.

The policewoman has been on restricted duties and is one of the four officers previously issued with a regulation 15 notice informing them they were being investigated for possible misconduct.

Scotland Yard said it expected the officer to be suspended later on Wednesday.

The plebgate row ignited when Mitchell was accused of a heated rant against officers as he left Downing Street on 19 September.

The Daily Telegraph published a police log of the incident which claimed he called officers "plebs" and swore at them repeatedly.

He insisted he did not use the words attributed to him, and later said he was the victim of a deliberate attempt to "toxify" the Tories and ruin his career.

An email from a civilian witness backing up the police account of events has also since been called into question.

Thirty detectives have taken statements from all 800 officers in the diplomatic protection group as part of the inquiry, which has taken nine months and cost taxpayers more than £140,000.